SEDIMENTARY PROVENANCE OF THE UPPER CAMBRIAN WORM CREEK QUARTZITE, IDAHO, USING U-PB AND LU-HF ISOTOPIC ANALYSIS OF ZIRCON GRAINS

The Worm Creek Member of the Upper Cambrian St. Charles Formation is an anomalous feldspathic sandstone within the Cordilleran carbonate platform in southeast Idaho. It contains two or three siliciclastic to carbonate cycles ranging from 5 to over 150 meters thick that each contain a basal feldspathic sandstone and an upper limestone with varying amounts of quartz sand. In its thickest northwestern exposures within the Paris thrust plate, the lowest interval is a very fine-grained, K-feldspar-rich (40%), arkosic sandstone, whereas the second and uppermost intervals are variably mixed carbonate and sub-arkosic sandstones. Detrital zircons within the Worm Creek display a strong unimodal 500 Ma age peak. This age population is also recognized in the Upper Cambrian DuNoir Limestone at Wind River Canyon, WY and the Pilgrim Formation at Melrose, MT.

U-Pb zircon samples from the Beaverhead and Deep Creek syenitic plutons yield ages of 505-490 Ma and are statistically identical to the age of detrital zircons in the Worm Creek Member. X-ray diffraction analysis suggests microcline is the main feldspar in the Beaverhead and Deep Creek plutons, and the detrital feldspar in Worm Creek sandstones, indicating derivation from hypabyssal feldspar-phyric plutons or porphyritic flow-domes. Four samples from the Beaverhead and Deep Creek plutons yielded zircon εHf values between -6.3 and 2.7, suggesting an isotopically intermediate source. εHf values of zircons for the 500 Ma peaks of six sandstones from the Worm Creek Member from the northern Bannock Range and the Bear River Range in southeast Idaho have similar intermediate εHf signatures with a range of -8.0 to 5.4.

Evidence of active Cambrian magmatism and exhumation of the central Idaho Lemhi Arch requires syndepositional plutonism during formation of the Cordilleran “passive” margin. The overlap of zircon εHf values in the plutons and the sandstones suggests that Beaverhead-belt plutons were emplaced and rapidly exhumed, sourcing the 505-490 Ma detrital zircon population in the Worm Creek Member. This exhumation event and subsequent first cycle deposition may be related to the Sauk II to III sea-level drawdown in the northern Rocky Mountains.